1. Field of The Invention a This application pertains to a rotor system for horizontal-axis wind turbines and to a blade structure, a rotor hub design and a pitch control system which enhance stability and improve the efficiency of horizontal-axis wind turbines.
2. Prior Art
A variety of wind turbine designs have been used to extract energy from wind, including both horizontal-axis and vertical-axis turbine systems. In typical horizontalaxis turbines, a nacelle enclosing power-transmitting mechanisms, electrical equipment and supporting a rotor system at one end, is mounted on a vertical tower. Rotor systems for horizontal-axis wind turbines ordinarily include one or more blades attached to a rotor hub which turns a power-transmitting shaft in the nacelle. The nacelle, bearing the rotor system, typically pivots about the vertical tower to take advantage of wind from any direction. The pivoting about this vertical-axis in response to changes in wind direction is known as yaw or yaw response and the vertical-axis is commonly referred to as the yaw-axis. Horizontal-axis turbines include upwind turbines and downwind turbines. The blades of a downwind turbine rotor system are contacted by wind after the wind travels past the tower and nacelle while the blades of an upwind turbine rotor system are contacted by wind before the wind passes the tower and nacelle.
The blade cross-section is often aerodynamic and may be based upon any airfoil configuration that enhances the efficiency of the blade. As wind moves past the blades with enough speed to generate sufficient lift to overcome inertial and drag forces, the rotor system rotates and the wind turbine converts the wind energy into electrical or mechanical energy for performing useful work.
Effective use of horizontal-axis wind turbines has been hindered by a number of problems, including excessive vibration and inadequate ability to position the rotor system properly relative to the mean wind direction. When a rotor system is not properly positioned with reference to the mean wind direction, the efficiency of the rotor system is significantly reduced. Proper positioning requires that the axis of rotor rotation be as nearly parallel to the mean wind direction as possible. When an angle of separation develops between the mean wind direction and the axis of rotor rotation, the power output of the rotor system, and therefore of the turbine, decreases. As the angle of separation increases, the decrease in power output is proportionally greater.
Conventional rotor systems tend to move unstably in response to changes in mean wind direction during operation by hunting for a proper yaw position relative to a new mean wind direction, rather than stably tracking such changes. Transient wind direction changes or wind gusts pivot the rotor system of typical wind turbines away from a proper yaw position and the system then hunts for a proper position relative to the mean wind direction when the transient wind dissipates. Hunting motions involve the back and forth movement of the axis of rotor rotation through the mean wind direction cyclically creating undesirable separation angles. For good yaw response a rotor system should stably track changes in mean wind direction rather than hunting for the proper position and should minimize hunting motions in response to transient wind direction changes
In addition to decreasing the power output by causing separation angles, unstable hunting motions result in undesirable vibration and stress. Blade fatigue and ultimate failure of the blade near its root is directly related to the number of hunting motions and the speed at which they occur. Rapid changes in yaw dramatically increase the forces acting against the rotational inertia of the entire rotor system, magnifying the bending moments at the blade root. Over time, additional stress cycles caused by hunting motions weaken blades near the blade root resulting in blade fatigue, decreasing equipment life and dependability.
Mechanisms for controlling yaw and yaw rate have been devised but none have provided adequate economical solutions to the problems associated with inadequate yaw response and stability. For instance, upwind turbines and some downwind turbines, use tailvanes which act as rudders to keep the rotor system positioned into the wind but these have not proven to be effective mechanisms for minimizing separation angle or the hunting movements that occur during yaw changes, particularly when used with downwind turbines. Electrically-powered yaw-drive systems have been used but these require additional energy and complex mechanisms that are subject to failure during operation. Some designs for downwind turbines position the aerodynamic center and the center of mass of the rotor system at a greater distance from the yaw axis than is needed for tower clearance in an attempt to cause the entire rotor system and the portion of the nacelle between the yaw-axis and the rotor system to behave like a tailvane. But increasing the distance between yaw-axis and the center of rotor mass magnifies bending moments on the blades during yaw and increases stress and vibration on the tower and rotor system. Excessive vibration and stress require more massive tower design and cause fatigue in the rotor hub and blade root thereby decreasing the useful life of the equipment and reducing dependability.
Many other sources of vibration hinder effective use of horizontal-axis wind turbines. For instance, blade motion in response to gyroscopic forces, wind shear, wind gusts and even blade balance, results in vibrations and cyclic motions not only in the blades, but in the rotor hub, the tower, bearings, and other components of a wind turbine as well. These vibrations and blade motions reduce the life and reliability of the affected components, and the performance of the equipment. This in turn decreases the cost-effectiveness of the wind turbine and its economic appeal.
Tower shadow is another source of destructive vibration that has attended downwind horizontal-axis wind turbines. Tower shadow refers to turbulence in air flow and a general reduction in wind velocity caused by the interference of the tower as the wind passes by. In downwind turbines, tower shadow causes each blade to encounter turbulence and lower wind velocity, and therefore lesser wind forces, as a blade moves through the sector of its rotation behind the tower. As a blade moves through the sector of its rotation above the tower, where the air flow has not been disrupted by the tower, less turbulence, higher wind velocity, and greater wind forces are encountered. Alternatively subjecting blades to greater and then lesser wind forces combined with lesser and then greater turbulence, results in unwanted vibrations and fluctuating stresses which worsen as the obstruction presented by the tower increases.
Blade design is another factor affecting the vibrations and stresses to which a wind turbine is subjected. Blades are usually load-bearing airfoils attached to a rotor hub, each blade being independent of the others with no external supporting members. These blades may be made of materials which allow bending and torsion of the load-bearing airfoil, but such materials are often expensive, increasing the cost of wind turbines. Uneven deflection of load-bearing airfoils can cause imbalance of the blades and rotor system resulting in greater vibration and stress on the entire turbine and tower structure. Some rotor designs use a slight coning angle at the blade root, usually 3-5 degrees, in an attempt to balance centrifugal and thrust forces which act upon a spinning rotor. Other systems, primarily upwind turbines, use no coning angle but may use struts strong in tension to connect each blade to a common point at some distance upwind of the rotor blades. These struts help prevent the wind thrust forces from snapping the blade at its root or forcing the blades into the tower. For some downwind turbine systems, rigid struts strong in compression have been used to connect each blade near its midpoint to a common hub some distance downwind of the rotor blades to resist the snapping action of wind and aerodynamic thrust forces, but such struts are usually not used with downwind systems since the wind and aerodynamic thrust forces act to increase clearance between the tower and the rotor blades.